Thermoelectric module

ABSTRACT

A thermoelectric module comprising an N-type thermoelectric element having excellent characteristics in atmospheric air even when the temperature rises to a medium-to-high temperature region of about 500° C. and, further, improving the conversion efficiency of a thermoelectric module, by the combination of an excellent P-type thermoelectric material and an excellent n-type thermoelectric material containing a compound having a skutterudite structure, the module comprising an N-type thermoelectric elements each containing a compound having a skutterudite structure, P-type thermoelectric elements each connected directly or by way of a metal member to the N-type thermoelectric elements and containing an Mn—Si series compound,

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention concerns a thermoelectric module for generatingelectric power by utilizing the temperature difference and, vice versa,generating temperature difference in accordance with electric powerapplied.

[0003] 2. Description of the Related Art

[0004] The thermoelectric module comprises a combination of P-type andN-type thermoelectric elements that utilize thermoelectric effects suchas Thomson effect, Peltier effect and Seebeck effect, and includesthermo-couples or electronic cooling elements. Since the thermoelectricmodule is simple in the structure easy to handle and can maintain stablecharacteristics, it has been noted for the use in a wide range ofapplications. Particularly, as the electronic cooling element, sincethis can conduct local cooling or precision temperature control near theroom temperature, research and development have been progressedgenerally for the application use to temperature control ofopto-electronics devices or semiconductor lasers, as well as tosmall-sized refrigerators.

[0005] The performance index Z representing the performance of athermoelectric element is expressed by the following equation usingspecific resistivity ρ, heat conductivity κ, Seebeck coefficient(thermoelectric performance) α:

Z=α ²/ρκ  (1)

[0006] In the equation, the Seebeck coefficient α takes a positive valuein the P-type element and a negative value in the N-type element. Alarger performance index Z is desired for the thermoelectric element.

[0007] Further, the maximum value η_(max) for the conversion efficiencyof the thermoelectric element is represented by the following equation.$\begin{matrix}{\eta_{m\quad a\quad x} = {\frac{\Delta \quad T}{T_{h}}\frac{M - 1}{M + \frac{T_{c}}{T_{h}}}}} & (2)\end{matrix}$

[0008] where T_(h) is a temperature on the high temperature side, T_(c)is a temperature on the lower temperature side and the difference of thetemperature ΔT is represented by the following equation:

ΔT=T _(h) −T _(c)   (3)

[0009] Further M is defined by the following equations (4) to (7).

M={square root}{square root over (ZT)}+1  (4)

{overscore (ZT)}={overscore (Z)}×{overscore (T)}  (5)

[0010] $\begin{matrix}{\overset{\_}{Z} = \frac{\int_{T_{c}}^{T_{h}}{Z\quad {T}}}{\Delta \quad T}} & (6) \\{\overset{\_}{T} = \frac{T_{c} + T_{h}}{2}} & (7)\end{matrix}$

[0011] By the way, as the material used for the P-type thermoelectricelement, Mn—Si series materials disclosed in the paper article of V. K.Zaitsev [Thermoelectric Properties of Anisotropic MnSi_(1.75)](Thermoelectric Engineering Handbook, p299-309, published from CRC in1995) have high conversion efficiency of about 11% at 650° C. Further,the report by Matsubara [Situation of Study on Skutterudite SeriesThermoelectric Materials] (Proceedings of Thermoelectric ConversionSymposium '97 (TEC '97), Thermoelectric Conversion Study Group, Jul. 25,1997) discloses the use of compounds having the skutterudite structureas the material for the P-type element. The term “skutterudite” isderived from minerals CoAs₃ produced in Skutterud, that is, the name ofa district in Norway. The literature describes that CoSb₃, RhSb₃ andIrSb₃ having the skutterudite structure are P-type semiconductors havinginherent band structures and carrier transport characteristics and havea feature in that the hole mobility is as large as from 2000 to 3000cm²/Vs at a room temperature. P-type Zn₄Sb₄ used so far is fragile andthe usable temperature is low. Ce(FeCo)₄Sb₁₂ is also fragile and tendsto be oxidized in atmospheric air at 500° C. or higher. Further, SiGeseries and FeSi series materials involve a problem of low performanceindex.

[0012] On the other hand, for constituting an excellent thermoelectricmodule, not only excellent P-type thermoelectric element but alsoexcellent N-type thermoelectric element are required. Heretofore,Mg—Si—Sn series, SiGe series, FeSi series, Pb—Te series or Pb—Se seriesmaterials have been used as the N-type thermoelectric element.

[0013] However, N-type Mg₂(Si—Sn) tends to be oxidized in atmosphericair at a temperature of 500° C. or higher. Pb—Te series or Pb—Se seriesmaterials have a worry of giving undesirable effects on environments.Further, the SiGe series and FeSi series materials involve a problem oflow performance index.

SUMMARY OF THE INVENTION

[0014] In view of the above, this invention intends to provide athermoelectric module using N-type thermoelectric elements havingexcellent characteristics in atmospheric air even when the temperaturerises to a medium-to-high temperature region of about 500° C. and,further, to improve the conversion efficiency of a thermoelectric moduleby the combination of an excellent P-type thermoelectric material and anexcellent n-type thermoelectric material.

[0015] For solving the foregoing problem, this invention provides athermoelectric module comprising N-type thermoelectric elements eachcontaining a compound having a skutterudite structure and P-typethermoelectric elements each containing an Mn—Si series compound andconnected directly or through a metalic component to the N-typethermoelectric elements.

[0016] This invention also provides a thermoelectric module comprisingan insulator in which a plurality of openings are formed in a latticeconfiguration, N-type thermoelectric elements each containing a compoundhaving a skutterudite structure and disposed in a first opening of theinsulator, P-type thermoelectric elements each containing an Mn—Siseries compound and disposed in a second opening of the insulator, and ametal member for connecting the N-type thermoelectric element and theP-type thermoelectric element.

[0017] According to this invention, since a compound having askutterudite structure is used for the N-type thermoelectric element,excellent characteristics can be obtained in atmospheric air even at atemperature of around 500° C. As the material for the N-typethermoelectric element, for example, Co—Sb series compound can be used.Further, when the Mn—Si series compound is used as the material for theP-type thermoelectric element, the conversion efficiency of athermoelectric module can be improved by the combination of theexcellent P-type thermoelectric material and the excellent N-typethermoelectric material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a view illustrating a thermoelectric module Related to afirst embodiment of this invention;

[0019]FIG. 2 is a view illustrating a modified example of athermoelectric module related to the first embodiment of this invention;

[0020]FIG. 3 is a view for assembling a thermoelectric modulemanufactured by thermal spraying;

[0021]FIG. 4 is a perspective view showing an example of an arrangementof P-type elements and N-type elements in the thermoelectric moduleshown in FIG. 3;

[0022]FIG. 5 is a perspective view showing a portion of thethermoelectric module in FIG. 3 along manufacturing process;

[0023]FIG. 6 is a view illustrating a power generation unit using thethermoelectric module related to the first embodiment of this invention;

[0024]FIG. 7 shows a model of a crystal structure of CoSb₃ used in thefirst embodiment of this invention;

[0025]FIG. 8 is a phase diagram of a Co—Sb series compound used in anN-type element;

[0026]FIG. 9 is a phase diagram of a Mn—Si series compound used in aP-type element;

[0027]FIG. 10 is a view illustrating a thermoelectric module related toa second embodiment of this invention;

[0028]FIG. 11 is a graph showing the performance index of thethermoelectric element used for the thermoelectric module related to thesecond embodiment of this invention;

[0029]FIG. 12 is a view illustrating the thermoelectric module relatedto a third embodiment of this invention; and

[0030]FIG. 13 is a graph showing the thermoelectric conversionefficiency of the P-type element and the N-type element used in each ofthe embodiments of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0031] A preferred embodiments of this Invention Is Illustrated in theaccompanying drawings.

[0032]FIG. 1 illustrates a thermoelectric module according to a firstembodiment of this invention.

[0033] A thermoelectric module 1 comprises, for example, two ceramicsubstrates 30 and 40 as heat exchange substrates. A P-type element(P-type semiconductor) 50 and an N-type element (N-type semiconductor)60 are connected intervention metallic component, for example, anelectrode 70 between the two ceramic substrates 30 and 40, to form PNelement pair. The metal member includes electrodes made of copper ornickel, a stress relaxation layer, a joining layer or a diffusion layer.For joining the thermoelectric element and the metal member, solderingmay be used in a case of a thermoelectric module for low temperatureusing copper as the metal member but they should be joined, in othercases than described above, by brazing, thermal spraying, solid-statewelding (sintering), vapor deposition or mechanical clamping.

[0034] Lead wires 80 are connected to the N-type element at one end ofthe PN element pair and the P-type element at the other end thereof.When the ceramic substrate 40 is cooled by cooling water or the like andthe ceramic substrate 30 is heated, an electromotive force is generatedto cause a flow of current as shown in FIG. 1. That is, electric powercan be taken out by making a temperature difference between both sidesof the thermoelectric module 1 (upper and lower sides in the drawing).

[0035] In FIG. 1, the P-type elements and the N-type elements areconnected by using electrodes formed on the two ceramic substrates but,as described below, the electrodes or the substrates may be omitted inwhole or in part. This will be explained with reference to FIG. 2.

[0036]FIG. 2A shows an example of connecting a P-type element 51 with anN-type element 61 not using an electrode. As shown in the figure, byforming a recess to each portion of the P-type element 51 and the N-typeelement 61, a current can be prevented from and flowing through shortcircuit. In this case, electrodes on both sides or one side can beomitted and substrates on both sides or one side can also be omitted.

[0037]FIG. 2B shows an example of connecting a P-type element 52 and anN-type element 62 by using a metal member, for example, electrode 72 onone side (lower side in the drawing) and connecting the P-type element52 and the N-type element 62 by not using an electrode on the other side(upper side in the drawing). Also in this constitution, a recess isformed to each portion of the P-type element 52 and the N-type element62. In this case, electrode on one side can be saved and the substrateon one side can also be saved.

[0038]FIG. 2C also shows an example of connecting a P-type element 53and an N-type element 63 by using a metal member, for example, anelectrode 73 on one side and connecting the P-type element 53 with theN-type element 63 without the electrode on the other side. In thisconstitution, each of the P-type element 53 with the N-type element 63has a curved shape. Also in this case, electrode on one side can besaved and the substrate on one side can also be omitted.

[0039] In the explanation of FIGS. 2A to 2C, when the P-type element andthe N-element are connected without using the electrode, thisconstitution includes both a case of directly connecting the elements toeach other and a case of connecting them by way of a joining layer or adiffusion prevention layer between the elements.

[0040] Manufacture of the thermoelectric module by thermal spraying isto be described with reference to FIG. 3 and FIG. 4. FIG. 3 is anassembly drawing of a thermoelectric module manufactured by thermalspraying. P-type elements 54 and N-type elements 64 are arranged asshown in FIG. 4 in a lattice 100 of an insulator provided with a leveldifference. A metal member 74 or 90 is made by thermal spraying thereon.

[0041] The manufacturing processes for the thermoelectric module bythermal spraying is to be explained with reference to FIG. 5.

[0042] At first, as shown in FIG. 5A, a level different lattice 100 isprepared from an insulator such as alumina ceramics. Then, as shown inFIG. 5B, P-type elements 54 and N-type elements 64 are arranged in theopening of the insulator lattice 100. Then, as shown in FIG. 5C, a metalmember 90 is made by thermal spraying so as to connect adjacentelements. Further, as shown in FIG. 5D, a metal member 74 is made bythermal spraying from above the metal member 90 to form a metal layer ofsingle or plural layer structure.

[0043] In the thermoelectric module, since the lattice is applied withlevel difference as shown in FIG. 5A, the adjacent elements areconnected by the metallic component made by thermal spraying as shown inFIG. 5C.

[0044] When the thermoelectric module as explained above is used as an(electric) power generation unit, as shown in FIG. 6, for example, acooling pipe 2 is arranged on one ceramic substrate of a thermoelectricmodule 1 (ceramic substrate on low temperature side). The cross sectionof the cooling pipe 2 perpendicular to longitudinal direction has acircular or rectangular shape, and the ceramic substrate on the lowtemperature side is cooled by flowing cooling water in the pipe. On theother hand, a metal plate 3 for absorbing heat is arranged to the otherceramic substrate of the thermoelectric module 1 (ceramic substrate onhigh temperature side). The heat collecting effect can be enhanced byintegrally forming heat collecting fins 4 to the metal plate 3 or (by)separately forming and connecting them. Further, a plurality ofthermoelectric modules 1 are connected by lead wires 5. Electric powercan be taken out from the thermoelectric modules by the temperaturedifference between the high temperature side and the low temperatureside.

[0045] In this invention, a compound having a skutterudite structure isused as the material for the N-type element. Particularly, the compoundhaving the following composition is suitable as the material for theN-type element.

[0046] (1) Compound Represented by M_(1-A)M′_(A)X_(B)

[0047] M represents one of Co, Rh and Ir, and M′ represents a dopant toform an N-type, which represents one of Pd, Pt, PdPt, X represents oneof As, P and Sb. Those satisfying the conditions: 0<A≦0.2 and 2.9≦B≦4.2are suitable. Particularly, a compound of a simple compositional ratiocan be obtained at B=3. Specific examples can include Co—Sb seriescompounds, for example, Co_(0.9)(PdPt)_(0.1)Sb₃. Explanation is to bemade with reference to FIG. 7 illustrating a crystal structure of Co—Sb₃having the same structure as Co_(0.9)(PdPt)_(0.1)Sb₃. This crystalstructure is referred to as a skutterudite structure. As shown in FIG.7, the unit lattice of CoSb₃ is a cubic lattice containing 32 atoms intotal of 8 Co atoms and 24 Sb atoms. The Co atom situates at the centerof the octahedron of Sb atoms formed by six Sb atoms. Eight octahedronsof Sb atoms are present in one unit lattice. The eight octahedrons formpolyhedra which has 20 planes of Sb atoms. Vacant cages where atoms arenot present are formed at the center and the corner of the unit lattice.

[0048] (2) Compound Represented by M(X_(1-A)X′_(A))₃

[0049] M represents one of Co, Rh and Ir, X represents one of As, P andSb and X′ represents one of Te, Ni and Pd. Those satisfying thecondition: 0<A≦0.1 are suitable.

[0050] (3) Compound Represented by M_(1-A)M′^(A)(X_(1-B)X′_(B))_(C)

[0051] M represents one of Co, Rh and Ir, M′ is a dopant for formingN-type and represents one of Pd, Pt and PdPt, X represents one of As, Pand Sb and X′ represents one of Te, Ni and Pd. Those satisfying theconditions: 0<A≦0.2, 0≦B≦0.1 and C=3 are suitable.

[0052] By using the materials as described above, a thermoelectricmodule using N-type elements hard to be oxidized in view of the propertyof the materials in atmospheric air even at a temperature around 500°C., having a relatively high strength and relatively mild to environmentcan be attained.

[0053] Among the materials described above, Co—Sb series compounds aredifficult to be manufactured and can not provide satisfactorycharacteristics easily since a single phase can not be obtained simplyas can be judged from the phase diagram of FIG. 8. In view of the above,device has been made of the manufacturing method in the manufacture ofthe thermoelectric element according to this embodiment. As a result oftrying various manufacturing methods, several manufacturing methodscomprising a combination of solid phase welding, explosive welding, MA(Mechanical Alloying), SPS (Spark Plasma Sintering), HP (Hot Press),annealing and plastic forming show actual effect for obtaining Co—Sbseries single phase compounds.

[0054] By the way, one or plurality of diffusion prevention layers arepreferably disposed as an intermediate layer between each of thethermoelectric elements or between the thermoelectric element and themetal member for preventing inter-diffusion of atoms. Since the linearexpansion coefficient is generally different between the materials usedfor the thermoelectric element and for the electrode when used in amedium-to-high temperature region around 500° C., one of them expandsmore to cause thermal stresses to be likely to peel the joined surfaceor form cracks in the thermoelectric element. In view of the above, thematerial used for the diffusion preventive layer is desirably selectedwhile taking the linear expansion coefficient thereof intoconsideration.

[0055] For example, when a Co—Sb series material is used for the N-typeelement and copper is used for the electrode, diffusion preventive layerhaving a linear expansion coefficient about greater than the linearexpansion coefficient of 8×10⁻⁶/K (800 K) of the Co—Sb series materialand about lower than the linear expansion coefficient of 20×10⁻⁶/K (800K) of copper is disposed. When the diffusion coefficient, heatconductivity, specific resistivity and Young's modulus are also takeninto consideration as the factors other than the linear expansioncoefficient, it is preferred to use Nb, V, Cr, Ti, Rh, Pt, Zr, W, Ta,Mo, Ni, Cu, Fe, Ag, Au, Sb and an alloy containing them as a firstintermediate layer (layer just above the thermoelectric element). Amongthem, Mo, Ta or Cr is particularly suitable.

[0056] The intermediate layer such as the diffusion preventive layer maycomprise a single layer or plural layers. When plural intermediatelayers are disposed, thermal stresses can be relaxed or destressed bygradually increasing the linear expansion coefficient of them from thethermoelectric element to the electrode.

[0057] As the electrode to be connected electrically with the N-typeelement, Ag, Al, Au, Co, Cu, Fe, Pt, Ti, Zn, Ni and alloy containingthem is used preferably. Cu, Ni and Fe are particularly preferred. Thelinear expansion coefficients of them are respectively 16.5×10⁻⁶/K,13.3×10⁻⁶/K and 11.7×10⁻⁶/K at room temperature, for example, which aresmaller when compared with the linear expansion coefficient of Al(23.9×10⁻⁶/K), and near the linear expansion coefficient of thethermoelectric element.

[0058] Further, an electrode holding layer may be disposed on thesurface of the electrode opposite side of the thermoelectric element. Itis preferred that the electrode holding layer is formed of a material ofsmaller linear expansion coefficient. The thermal stress can be reducedby sandwiching the electrode between an intermediate layer having smalllinear expansion coefficient and the electrode holding layer havingsmall linear expansion coefficient. As the material for the electrodeholding layer, ceramics using Al₂O₃ or AlN as the raw material, glassusing SiO₂ as a raw material, Cr, Nb, Pt, Rh, Si, Ta, Ti, V, Mo, W, Zrand an alloy containing them can be used.

[0059] The thickness of the intermediate layer is preferably about atthe same level as with the thickness of the electrode (1:1).Specifically, the thickness for the intermediate layer or the electrodeis preferably from 5 μm to 1000 μm and, further preferably, from 50 μmto 300 μm. As described previously, the intermediate layer may be amulti-layered structure. It may also be constituted such that theelectrode sandwiched between the intermediate layer and the electrodeholding layer.

[0060] On the other hand, as the material for the P-type element, Mn—Siseries compounds are used this embodiment. Particularly, compoundshaving the following compositions are suitable as the materials for theP-type element.

[0061] (1) MnSi_(A)

[0062] where 1.72≦A≦1.75.

[0063] (2) Compounds in which one or more of Ge, Sn, Mo and Al is addedby 0 to 5 atm % as the dopant to MnSi_(A).

[0064] For example, Mn_(1-B)Mo_(B)Si_(A-C-D)Ge_(C)Al_(D) can bementioned. The relation may be 0<B, C, D≦0.1.

[0065] Also for the Mn—Si series compounds used as the material for theP-type element, since a single phase can not be obtained easily as canbe seen from the phase diagram in FIG. 9, they are difficult to bemanufactured and can not provide satisfactory characteristics easily.Then, several manufacturing methods comprising a combination of methodssuch as solid phase welding, explosive welding, MA, SPS, HP, annealingand plastic forming are effective also when the material for the P-typeelements is manufactured.

[0066] Like the N-type element, it is also desirable for the P-typeelement to dispose one or plural of diffusion preventive layers betweeneach of the thermoelectric elements or between the thermoelectricelement and the metal member.

[0067] For example, when the Mn—Si series material is used for theP-type element and copper is used for the electrode, a diffusionpreventive layer having a linear expansion coefficient of greater thanabout the linear expansion coefficient of the Mn—Sn series material of12×10⁻⁶/K (800 K) and lower than about the linear expansion coefficientof copper of 20×10⁻⁶/K (800 K). When the diffusion coefficient, heatconductivity, specific resistivity and Young's modulus also taken intoconsideration as the factors other than the linear expansioncoefficient, it is preferred to use Nb, V, Cr, Ti, Rh, Pt, Zr, W, Ta,Mo, Ni, Cu, Fe, Ag, Au, Si and an alloy containing them as a firstintermediate layer (layer just above the thermoelectric element). Amongthem, Nb. Mo, Ta, Pt or Cr is particularly suitable.

[0068] Further, the intermediate layer such as the diffusion preventivelayer described above may be a single layer or multiple layers. Whenmultiple intermediate layers are disposed, the thermal stress can berelaxed by gradually increasing the linear expansion coefficient of themfrom the thermoelectric element to the electrode.

[0069] The electrode connected electrically with the P-type element caninclude, like the N-type element, Ag, Al, Au, Co, Cu, Fe, Pt, Ti, Zn, Niand an alloy containing them and, Cu, Ni and Fe are preferredparticularly.

[0070] The thermal stress can be reduced by making the shape of theP-type and the N-type thermoelectric elements as a columnar, cylindricalor trapezoidal shape. Further, the thermal stress can be reduced bychamfering edges of a thermoelectric element having the columnar,cylindrical, trapezoidal or rectangular (solid) shape.

[0071] Further, an electrode holding layer may be disposed on thesurface of the electrode opposite to the thermoelectric element. It ispreferred that the electrode holding layer is formed of a material ofsmall linear expansion coefficient. The thermal stress can be reduced bysandwiching an electrode between an intermediate layer having smalllinear expansion coefficient and the electrode holding layer havingsmall linear expansion coefficient. As the material for the electrodeholding material, ceramics using Al₂O₃ or AlN as the raw material, glassusing SiO₂ as the raw material, Cr, Nb, Pt, Rh, Si, Ta, Ti, V, Mo, W, Zrand an alloy containing them can be used.

[0072] The thickness of the intermediate layer is preferably about thesame level as the thickness of the electrode (1:1). Specifically, thethickness for the intermediate layer or the electrode is preferably from5 μm to 1000 μm and, further preferably, from 50 μm to 300 μm. Asdescribed previously, the intermediate layer may be a multi-layeredstructure. It may be constituted such that the electrode may besandwiched between the intermediate layer and the electrode holdinglayer.

[0073] When a diffusion preventive layer is formed of an element whichis identical for the P-type element and the N-type element, thediffusion preventive layer can be formed simultaneously for the P-typeelement and the N-type element, for example, by thermal spraying, whichsimplifies the manufacture of the thermoelectric module. For example,when CoSb₃ is used for the N-type element and MnSi_(1,73) is used forthe P-type element, V, Cr, Ti, Nb, Fe, Cu, Ni, Ta, W, Zr or Mo is usedfor the diffusion preventive layer and Cu, Ni, Fe or the like ispreferably used for the electrode.

[0074] As the process of joining the thermoelectric element,(forexample,) with an electrode, thermal spraying is particularly superior.For the joining process in a case of using the Co—Sb series compound forthe thermoelectric element, the methods including brazing, thermalspraying, solid phase welding and vapor deposition were studied. As aresult, vapor deposition involves a problem that the degree of adhesionis lowered. Further, in the case of brazing, while brazing materialshaving a melting point of about 600° C. is preferred for the Co—Sbseries compound, there are only few brazing materials having suchtemperature region. Further, since melting point is different betweenthe Co—Sb series compound used for the N-type element and the Mn—Siseries compound used for the P-type element, if the brazing material isselected conforming the Co—Sb series compound of low melting point (forexample, CoSb₃), the efficiency of the Mn—Si series compound can not beprovided. The solid phase welding is suitable to a case of usingmaterials having strength and melting point close to each other for theN-type and P-type thermoelectric elements but it is difficult to findjoining conditions adaptable to both of the Co—Sb series compound andthe Mn—Si series compound defining which are different from each otherboth in the strength and the melting point. On the contrary, accordingto thermal spraying, a great amount of thermoelectric elements,electrodes and the like can be joined at once in a low temperaturecircumstance and high strength can be obtained.

[0075] On the other hand, when a thermoelectric module is assembled byusing brazing or solid phase welding, diffusion preventive layers can beformed of different elements for the P-type element and the N-typeelement. In this case, the diffusion preventive layer is formed, forexample, by plating. For example, when CoSb₃ is used for the N-typeelement and MnSi_(1.73) is used for the P-type element, it is preferredto use Nb, Ni, Fe, Cr, Sb, Ti, Mo, Zr, Cu, W, Ta and an alloy-containingthem as the diffusion preventive layer and use Cu, Ni, Fe or the like asthe electrode. On the other hand, in the P-type element, it is preferredto use Cr, Ni, Fe, Si, Mo and an alloy containing them as the diffusionpreventive layer and use Cu, Ni, Fe or the like as the electrode. Alsoin this case, same as the case of thermal spraying, the intermediatelayer such as the diffusion preventive layer may be formed as amulti-layered structure or it may be structured to sandwich theelectrode between the intermediate layer and the electrode holdinglayer.

[0076] Then, a second embodiment according to this invention is to bedescribed. In this embodiment, each of N-type and P-type thermoelectricelements is formed as a segment type of a multi-layered structure.

[0077]FIG. 10 shows a thermoelectric module related to this embodiment.FIG. 10 shows an example in which each of N-type and P-typethermoelectric elements has a two layered structure. CoSb₃ with additionof an N-type dopant is used for an N-type thermoelectric element 65 onthe high temperature side, and an N-type Bi—Te series compound is usedas an N-type thermoelectric element 66 on the low temperature side. Onthe other hand, an Mn—Si series compound with addition of a P-typedopant is used as a P-type thermoelectric element 55 on the hightemperature side and a P-type Bi—Te series compound is used as a P-typethermoelectric element 56 on the low temperature side. When thethermoelectric element on the high temperature side and thethermoelectric element on the low temperature side are connected, theelements may be connected directly with each other or may be connectedby way of a junction layer or a diffusion preventive layer between theseelements.

[0078] As described above, different materials are used for the hightemperature side and the low temperature side, because the temperaturecharacteristic of the performance index Z differs depending on thematerial, and suitable material is present for each of the temperaturesas shown in FIG. 11. With such a constitution, the thermoelectricconversion efficiency of the thermoelectric module can be improved morecompared with the case of forming the entire thermoelectric element witha single material.

[0079] The N-type thermoelectric element 65 on the high temperature sideand the P-type thermoelectric element 55 on the high temperature sideare connected by way of an electrode 71. Further, when the N-typethermoelectric element 66 on the low temperature side and the P-typethermoelectric element 56 on the low temperature side are connected byway of an electrode 72 to a load resistance R, electric power dependingon the temperature difference between the high temperature side and thelow temperature side can be taken out. Connection between thethermoelectric element and the electrode is same as that explained forthe first embodiment.

[0080] Then, a third embodiment according to this invention is to beexplained. In this embodiment, the unit for the thermoelectric modulehas a multi-layered structure and is formed into a cascade type.

[0081]FIG. 12 shows a thermoelectric module according to thisembodiment. FIG. 12 shows an example in which the unit of thethermoelectric module is constituted as a two layered structure. In themodule unit on the high temperature side, CoSb₃ with addition of anN-type dopant is used as an N-type thermoelectric element 67 and anMn—Si series compound with addition of a P-type dopant is used as aP-type thermoelectric element 57. On the other hand, in the module uniton the low temperature side, an N-type Bi—Te series compound is used asan N-type thermoelectric element 68 and a P-type Bi—Te series compoundis used as a P-type thermoelectric element 58. Different materials areused between the high temperature side and the low temperature sidebecause of the same reason as that for the second embodiment.

[0082] In the module unit on the high temperature side, the N-typethermoelectric element 67 and the P-type thermoelectric element 57 aredisposed between ceramic substrates 110 and 120 and connected by way ofan electrode 73. Further, in the module unit on the low temperatureside, the N-type thermoelectric element 68 and the P-type thermoelectricelement 58 on the low temperature side are disposed between ceramicsubstrates 120 and 130 and connected by way of an electrode 74.Connection between the thermoelectric element and the electrode is thesame as that explained for the first embodiment.

[0083] When such module units are connected to load resistances R1 andR2, electric power can be taken out depending on the temperaturedifference in each of the module units. Alternatively, the module unitsmay be connected in series or connected in parallel.

[0084] Then, characteristics of the N-type element and the P-typeelement used in the embodiments described above are to be explained.

[0085]FIG. 13 is a graph showing a thermoelectric conversion efficiencyof the Co—Sb-series N-type element and the Mn—Si series P-type elementat the element level. The abscissa indicates the temperature on the hightemperature side, the temperature on the low temperature side beingfixed to a room temperature (27° C.). The thermoelectric conversionefficiency of the Co—Sb series N-type element shows a excellent value ofabout 11% at the temperature difference of 550° C. Further, thethermoelectric conversion efficiency of the Mn—Si series P-type elementalso shows a favorable value of about 11% at the temperature differenceof 650° C.

[0086] Further, when the thermoelectric module is constituted as acombination of the Co—Sb series N-type element and the Mn—Si seriesP-type element as in the embodiments described above, the thermoelectricconversion efficiency of the thermoelectric module was about 10.5% atthe temperature difference of 600° C. The thermoelectric conversionefficiency described above shows a larger value when compared with thethermoelectric conversion efficiency of the Si—Ge series thermoelectricmodule (4.6% at the temperature difference of 500° C.) or thethermoelectric conversion efficiency of the cascade module of the Si—Geseries and the PbTe—GeTe series (9.9% at the temperature difference of687° C.). In addition, the thermoelectric module comprising thecombination of the Co—Si series N-type element and the Mn—Si seriesP-type element has an advantage in that it is mild to the environmentand obtainable at a reduced cost.

[0087] As has been described above according to this invention, it ispossible to provide a thermoelectric module of using the N-typethermoelectric element having excellent characteristics in atmosphericair even at a temperature of about 500° C. Further, the combination ofexcellent P-type thermoelectric material and excellent N-typethermoelectric material can improve the conversion efficiency of thethermoelectric module.

We claim:
 1. A thermoelectric module comprising N-type thermoelectricelements each containing a compound having a skutterudite structure,P-type thermoelectric elements each connected directly or by way of ametal member to the N-type thermoelectric elements and containing anMn—Si series compound,
 2. A thermoelectric module as defined in claim 1,wherein the N-type thermoelectric element contains a Co—Sb seriescompound.
 3. A thermoelectric module as defined in claim 1, wherein themetal member contains one of Cu, Ni and Fe.
 4. A thermoelectric moduleas defined in claim 2, wherein the metal member contains one of Cu, Niand Fe.
 5. A thermoelectric module as defined in claim 1, which furthercomprises at least one of an intermediate layer formed between theN-type thermoelectric element and the metal member, and an intermediatelayer formed between the P-type thermoelectric element and the metalmember.
 6. A thermoelectric module as defined in claim 2, which furthercomprises at least one of an intermediate layer formed between theN-type thermoelectric element and the metal member, and an intermediatelayer formed between the P-type thermoelectric element and the metalmember.
 7. A thermoelectric module as defined in claim 3, which furthercomprises at least one of an intermediate layer formed between theN-type thermoelectric element and the metal member, and an intermediatelayer formed between the P-type thermoelectric element and the metalmember.
 8. A thermoelectric module as defined in claim 4, which furthercomprises at least one of an intermediate layer formed between theN-type thermoelectric element and the metal member, and an intermediatelayer formed between the P-type thermoelectric element and the metalmember.
 9. A thermoelectric module as defined in claims 5, wherein theintermediate layer contains one of Mo, Ta and Cr when the intermediatelayer is formed between the N-type thermoelectric element and the metalmember, while the intermediate layer contains one of Nb, Mo, Ta, Pt andCr when the intermediate layer is formed between the P-typethermoelectric element and the metal member.
 10. A thermoelectric moduleas defined in claims 6, wherein the intermediate layer contains one ofMo, Ta and Cr when the intermediate layer is formed between the N-typethermoelectric element and the metal member, while the intermediatelayer contains one of Nb, Mo, Ta, Pt and Cr when the intermediate layeris formed between the P-type thermoelectric element and the metalmember.
 11. A thermoelectric module as defined in claims 7, wherein theintermediate layer contains one of Mo, Ta and Cr when the intermediatelayer is formed between the N-type thermoelectric element and the metalmember, while the intermediate layer contains one of Nb, Mo, Ta, Pt andCr when the intermediate layer is formed between the P-typethermoelectric element and the metal member.
 12. A thermoelectric moduleas defined in claims 8, wherein the intermediate layer contains one ofMo, Ta and Cr when the intermediate layer is formed between the N-typethermoelectric element and the metal member, while the intermediatelayer contains one of Nb, Mo, Ta, Pt and Cr when the intermediate layeris formed between the P-type thermoelectric element and the metalmember.